A recent development in telecommunications technology has been the introduction of the asynchronous transfer mode (ATM) transmission technique. The asynchronous transfer mode (ATM) technology is a flexible form of transmission which allows various types of service traffic, e.g. voice, video or data, to be multiplexed together on to a common means of transmission, the traffic being carried in cells each having a header indicating its destination. The service traffic is adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of the ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL). The technique allows large volumes of traffic to be handled reliably and efficiently.
A description of a cell multiplexing apparatus handling multiple items of information is given by Takashima et al. in specification No. U.S. Pat. No. 5,509,007. An arrangement for providing ATM call interworking with the PSTN is described by Zsehong et al. in IEEE/ACM Transactions on Networking, vol. 2, No. 1, 1 Feb. 1964, pages 30 to 39. Voice transport on an ATM broadband network is described by Covington et al. in Communications Technology for the 1990's and Beyond, Dallas, Nov. 27-30, 1989, vol.3 of 3, 27 Nov. 1989, IEEE, pages 1921 to 1925.
Various enhancements of the basic ATM transmission protocol have been proposed to accommodate specific user requirements. One of these enhancements has been the introduction of minicells for low bit rate users to reduce the cell assembly delays previously experienced by such users. In such a system, minicells from a number of users can be multiplexed together and packed into a standard ATM cell for transmission over a common virtual channel. A number of recommendations have been made for an adaptation layer to provide support of these services, but none of these has effectively accommodated the different requirements of the system users.
There are three major types of system user are detailed below.
Error Tolerant Low Bit-rate
Typically these services are used in mobile wireless applications for carrying traffic via a fixed network between wireless base stations and a mobile switching centre or between two mobile switching centres. Low bit rate coding schemes, usually involving compression, are used to transfer synchronous data (usually voice). The relatively high error rates associated with the air interface in a wireless system generally requires these services to use speech and channel coding algorithms that contain error protection (particularly over vulnerable speech parameter and control fields) and typically use ‘forward adaptation’ coding In a forward adaptation algorithm, the speech coding process operates entirely within one data packet. It is therefore memory-less, and an error associated with the loss or corruption of a data packet does not extend beyond the boundaries of that packet. Recovery mechanisms exist in the speech decoding process to mitigate the effects of lost or severely corrupted data packets, provided that the fact is signalled to the decoder. In the GSM system for example, the previous “good” packet may substitute for a corrupt packet. The basic requirements for this category are low mini-cell assembly delay together with a high bandwidth efficiency.
Error Intolerant Low Bit-rate
Low bit rate (usually involving compression) coding schemes are used to transfer synchronous data (usually voice). Typically these services are used in wire-line applications, and are expected to provide a high quality service over relatively low error-rate physical links. The speech coding algorithms employed do not contain significant error protection generally, and are often ‘backward adaptation’ algorithms.
In a backward adaptation algorithm the speech coding process runs over a number of samples such that information decoded in one sample directly influences the decoding of several others. The loss or corruption of a sample can lead to an error occurring over an extended time period due to the memory and adaptation time constants contained within the coding process.
Backward adaptation coders tolerate isolated bit or sample errors—for example in ADPCM a click and noise distortion may be audible—but they have no intrinsic mechanism to mitigate the effects of long bursts of consecutive sample errors caused by loss or corruption of a AAL-CU mini-cell. There are two options available for supporting these legacy services. The first option is the reduction of the end-to-end error rate by using forward error correction and interleaving data over several packets. This is a solution that is tuned to the transport medium and could support many service types. However, the complexity and increase in end-to-end delay that would result makes it unsuitable for the new AAL. The second option is the creation of a service specific error mitigation scheme which, based on reliable error indication scheme in the common part sub-layer, can carry out an appropriate recovery procedure in the convergence sub-layer, or by invoking mechanisms intrinsic to the service. This service category is therefore vulnerable to bit error, mini-cell loss or mis-concatenation/delivery and the ability to detect lost or corrupted mini-cells is a key requirement.
Mobile Packet Data
This is a recently introduced service which carries low bit rate data such as GSM data. The basic requirement for this category is high bandwidth efficiency. The service is delay tolerant and generally does not require any further protection in the AAL layer. However, the length of a data unit may be up to several hundred bytes long and a mechanism is therefore required to enable the data to be segmented. As a consequence, the loss or corruption of a mini-cell would cause the discard of the entire data packet generally. In protocols based on retransmission, a noisy transmission environment could become swamped.
The specific requirements of these different types of system user cannot all be met by current ATM transmission protocols. A particular problem with accommodation of these various users is that of determining the length of each minicell so the cells can be correctly delineated in the de-multiplexing process. The different user services will normally require the use of minicells of different lengths for each user. Further, some users may require variable length minicells. At present length determination or delineation of individual minicells is effected by the use of a length identifier (LI) field which is used to encode the explicit length of the minicell protocol data unit. The provision of this information occupies bandwidth that might otherwise be made available for revenue earning payload use or for the provision of other control functions.